Various expressions, such as Boolean and arithmetic expressions, are used in computer technology. Many search engines, for example, use Boolean expressions as search terms. More lengthy and complex rules, for example those used by network security systems, are also commonly represented and created using Boolean and other expressions.
Visual tree representations are also common in computer technology. For example, directory trees are used as graphical user interfaces to display the organizational hierarchy of a directory. Expressions can also be displayed according to such a tree structure. Such a tree structure is sometimes referred to as an expression tree. Expression trees display expressions in prefix notation. Because of their inherent prefix nature, expression trees can be difficult for a human to read and edit.